The contact process for the production of sulfur trioxide in sulfuric acid manufacture involves the catalytic oxidation treatment of sulfur dioxide-containing gases from sulfur, or metallurgical or waste acid burning operations. This process uses many stages of heat transfer to colder unconverted gases between and after catalyst beds. In the classic single absorption plant process, there are typically three or four such heat exchange steps, while in the double absorption plant process four to six heat transfer steps may be used. Even in the case where elemental sulfur is the source and the plant uses double absorption, at least two such heat transfer steps are normally involved notwithstanding typically fewer operations involving heat transfer between process gases are involved.
The very large volumes of gases used in such contact process atmospheric pressure plants are difficult and expensive to remove from the catalytic converter and often designers have inserted heat exchangers within the converter to provide the necessary cooling between catalyst beds. The exchangers of interest in such converters have been axially located in a central core or similar portion of the converter with the exchanger being vertically and the catalyst beds horizontally disposed. In such internally combined heat exchange-converter units, the most favoured location for an exchanger is that after catalyst bed number one. Such an exchanger is, thus, known as the "hot exchanger" or "No. 1 exchanger". The hot exchanger transfers heat between the gas leaving the first catalyst bed and the gas entering the first bed. A second exchanger of note is the exchanger after bed 2 in a double absorption process where the heat between beds 2 and 3 is removed to heat the gas flowing to bed 4. The gases being heated in each case are generally already warm from previous exchange steps.
Heat transfer duty is best served using stainless steels as construction materials. With the trend towards use of stainless steels in newer converters, use of stainless steel in the hot exchanger or a reheat exchanger allows a single type of material to be used in the combined exchanger converter. A resultant welded construction is thus possible to offer good protection against leaks.
One prior art combination of a hot exchanger with a single catalyst bed has been used in a number of plants handling metallurgical source sulfur dioxide gases. In this case the catalyst bed is the first bed and the exchanger is the hot exchanger. In this arrangement, a radially symmetrical exchanger is installed vertically co-axial with the central axis of the single catalyst bed which surrounds the exchanger in the form of an annular bed. The incoming gas to the exchanger flows down the tubes of the exchanger and from the bottom thereof flows through a plenum between the shell of the vessel and the bottom of a basket containing the catalyst and the hot exit gas. At the periphery of the converter, the warmed incoming gas passes through an annulus around the catalyst bed to a ring with holes above the catalyst which allows the gas to enter the space above the catalyst. The gas then flows downward through the catalyst bed into a hot plenum and from here flows into the shell side of the exchanger. The gas then flows upward through the exchanger to a top discharge duct which conducts the gas to the next contacting operation.
However, notwithstanding the introduction of aforesaid catalytic converters having an internal heat exchanger in the contact process, there is a need for an improved internal heat exchanger-catalytic converter.